Orthodontic brackets that have been precoated with a resin-based adhesive have been provided for some years under the name "APC.TM. adhesive coated bracket" by 3M Unitek. There are some challenges related to resin-based adhesives, such as shelf life under normal storage conditions. Because the resin-based adhesive is a liquid, it is inherently unstable with time. The labile nature of certain of the chemical constituents, especially the light cure chemicals, as well as the volatility of certain components lead to gradual changes in the physical properties as well as in the ability to cure when exposed to visible light. A precoated bracket that has exceeded its shelf life may lead to a bond that is weak and may spontaneously debond.
Orthodontic brackets that have been precoated with a resin-based adhesive may also have a viscosity that is higher than optimum viscosity needed during application of the bracket to the tooth. This is because the adhesive, besides its primary role of adhering the bracket to the tooth, plays a secondary role of adhering the bracket to the release liner inside the blister in which it is packaged. To prevent the bracket from drifting or lifting off the liner, the adhesive may be made thicker than ideal. This may limit its acceptability to some extent among orthodontic professionals and their staffs and also, in general, may give weaker bonds than a less viscous material.
Recently, glass ionomer cements have been made stronger by modifying them with acrylic polymers and also adding light cure chemistry as well as acrylic dark cure chemistry. This has led to materials that are usable as orthodontic bracket adhesives. The major drawback remains the necessity to mix the powder and liquid at chairside just prior to bonding. The dark cure chemistry is such that there is only a few minutes of open time before the adhesive starts to set up and must be discarded.